Heretofore, for protection against overvoltage, varistors having a monolayer sintered body primarily composed of ZnO have been widely used. In recent years, in addition to the protection against overvoltage, as electrostatic discharge (ESD) protection elements and noise filters, multilayer varistors composed of a plurality of internal electrodes disposed in a sintered body have been increasingly used.
In addition, concomitant with the trend toward higher integration and lower drive voltage of electronic apparatuses such as mobile communication apparatuses and notebook personal computers, varistors have been strongly required to be stably driven at a lower rated voltage and to have superior reliability.
In electronic apparatuses, ESD may occur frequently at interface portions with the exterior, and as elements for protecting interior devices, a great number of Zener diodes and chip type varistors have been widely used. The chip type varistors have no polarity in current-voltage characteristics (I–V characteristics) and have bi-directional characteristics. Accordingly, as compared to SMD type Zener diodes incorporating two elements, when the chip type varistors are used, reduction in cost and reduction in mounting area can be achieved.
Incidentally, the threshold voltage (hereinafter referred to as “varistor voltage”) of varistors using a sintered body primarily composed of ZnO is proportional to the number of grain boundaries present between electrodes. It has been believed that the varistor voltage per one grain boundary is between 2 to 3 V. Accordingly, in order to form a varistor to be driven at a low voltage of 30 V or less, the number of grain boundaries present between electrodes must be not more than ten and several.
As a method for decreasing the number of grain boundaries between electrodes, there may be mentioned a method for decreasing the thickness of a characteristic layer (that is, the varistor layer) to decrease the number of grain boundaries and a method for increasing the grain diameter to decrease the number of grain boundaries. In the method for decreasing the thickness of a characteristic layer, due to the presence of pin holes or the variation in thickness of green sheets, which are sheets before a sintering step is performed for forming the characteristic layer, the properties may significantly vary in some cases, and in addition, grain strength may be decreased in some cases. On the other hand, in the method for increasing the grain diameter, the growth of grains must be facilitated, and hence abnormal growth of grains is liable to occur, thereby increasing the variation in grain diameter. As a result, the variation in properties may be increased in some cases.
Accordingly, when a multilayer varistor to be driven at a low voltage is formed, in order to maintain element strength and to decrease the variation in properties, the sintered body layer present between internal electrodes, that is, the characteristic layer must have a thickness at a certain level or more, and in addition, the variation in grain diameter must be decreased.
A varistor material primarily composed of ZnO is generally categorized into a material containing a Bi base sub-component formed of Bi2O3, Sb2O3, CoO, MnO, and the like disclosed, for example, in Japanese Examined Patent Application Publication No. 53-11076 and a material containing a Pr base sub-component formed of Pr6O11, CoO, and the like disclosed, for example, in Japanese Examined Patent Application Publication No. 56-11076.
By the use of the barrister material containing a Bi base sub-component, a varistor suitably used for overvoltage protection for a large current application can be easily supplied at a relatively low cost. However, in firing, Bi2O3 or Sb2O3 having a low melting point tended to form a liquid phase and was also liable to evaporate. As a result, it was difficult to decrease the variation in grain diameter. Accordingly, when the number of grain boundaries was decreased for realizing a lower drive voltage, the variation in properties was inevitably increased due to the variation in grain diameter. Consequently, it has been difficult to stably manufacture and supply multilayer varistors which can be driven at a lower voltage and which has superior reliability. In addition, since the variation in grain diameter was liable to increase, a surge current or ESD was concentrated at positions at which grains having a large grain diameter were present, and the resistances against surge current and ESD were also liable to decrease.
On the other hand, in the varistor material containing a Pr base sub-component, Bi2O3 and Sb2O3 are not contained which form a liquid phase at a low temperature and which are liable to evaporate. Accordingly, varistors having stable and superior properties can be manufactured in mass production and can be supplied. However, as compared to the varistor material containing a Bi base sub-component, the varistor material containing a Pr base sub-component had a problem of a large leak current. In order to realize a lower drive voltage, when the thickness of the characteristic layer is decreased, the leak current is further increased, and the insulation resistance and the voltage non-linearity are degraded. Hence, there have been problems in that the power consumption is increased and that the malfunction of signal circuits occurs. In order to decrease the leak current, it is effective that the donor concentration in ZnO grains be decreased or that a larger amount of an insulating material be added. However, when the methods described above are used, the surge resistant is significantly decreased.
When a conventional varistor material containing a Pr base sub-component is used, in a multilayer chip type varistor which can be driven at a low voltage of 30 V or less, it has been difficult to suppress the leak current and to realize a high surge resistance.
In Japanese Unexamined Patent Application Publication No. 7-29709, a voltage non-linear resistor has been disclosed which can be driven at a low voltage and which has a high surge resistance and a large resistance against electrostatic discharge. In this technique, a voltage non-linear resistor has been disclosed having a composition containing ZnO as a primary component, and Pr6O11, Bi2O3, Mn3O4, and CoO as sub-components. However, since Bi2O3 tends to form a liquid phase at a low temperature or is liable to evaporate, it has been difficult to obtain uniform particle diameters. In addition, it has also been difficult to stably supply a voltage non-linear resistor which has superior reliability and which can be driven at a low voltage.
In consideration of the present situation of the conventional techniques described above, an object of the present invention is to obtain a ceramic composition for a varistor and the varistor, the ceramic composition capable of forming a highly reliable varistor which can be stably driven at a low voltage and which has a small leak current, a high surge resistance, and a large ESD resistance.